Even on an acceptable substrate, that mastic would take a very long time to dry under 12" tiles. If I understand you correctly, they were set directly to vinyl. That's a no-no even with a proper portland-cement-based adhesive. Mastic may never dry between ceramic and vinyl. It's almost the same as keeping it in the bucket.
If your friend still wants tile on those floors, the vinyl will have to be removed. The surface of the cement slab has to be very flat, and relatively free of any prior adhesive material. Any cracks would have to be dealt with appropriately with an isolation membrane. Cracks with vertical displacement would likely be a deal-breaker for tile.
If you decide to pick those tiles up and set them back down properly, make sure you have a plan for putting any cut tiles back were they were - unless you have enough extras to make new cuts.
Originally Posted by cx
Looks like ol' Fast Fingers Dan hasn't anything better to do this morning, eh?
Nothing better to do, but plenty of stuff I should be doing - like installing a new kitchen sink faucet.